1. Field of the Invention
The present invention relates to mounting structures to the walls of enclosures used for isolating or sealing substances within the enclosures from the surrounding environment. More particularly, the present invention relates to window assemblies for such enclosures that provide improved sealing capabilities and facilitate installation and removal of windows thereof.
2. State of the Art
It is often necessary to work with or handle hazardous substances that are associated with many of today's industrial processes. Waste items contaminated with radiation, for example, may require sorting into separate categories before being disposed of. The manufacture of medical products may also require risk-associated handling procedures, such as in making radiopharmaceuticals used in cancer treatments or working with biologically active compounds. When working with these substances, it is essential to isolate them from operating personnel to protect them from harmful exposure. To this end, enclosures are used that typically comprise rigid-walled containers having access portals for insertion, removal and manipulation of the hazardous substances, as well as windows to allow observation of conditions within the container. Enclosure sizes may range from room-sized containers intended for holding and processing large quantities of material, down to small containers called “glove boxes” that have glove ports sealed with gloves for individually handling small quantities of a substance.
To prevent the spread of contamination, the access portals and windows of an enclosure are sealed in a leak-tight fashion. This is especially important in situations where there is a pressure differential between the interior of the enclosure and the surrounding environment, as when substances must be processed in a vacuum or at low pressure. In the case of radioactive substances, it is also necessary to form or line the enclosure walls, access portal covers and windows with shielding that inhibits radiation from escaping from the enclosure. Such radiation-shielded enclosures, sometimes referred to as “hot cells,” are commonly formed with walls and access portal covers comprising stainless steel lined with lead and windows impregnated with lead or other radiation-absorbing materials.
In response to the above-described structural requirements, various approaches have been developed for mounting and sealing windows within the walls of enclosures for containing hazardous substances. One widely used approach is a window assembly called a “zipper window.” FIGS. 1A and 1B show a glove box 2 constructed with a zipper window assembly. As seen in FIG. 1A, glove box 2 includes top, bottom, and side walls 4 formed from a material such as plates of stainless steel. Stainless steel is desirable for forming walls 4 because it is resistant to corrosion or other reactive effects that may be associated with substances contained within glove box 2. Stainless steel also has a smooth surface finish that is easy to clean, which is important to prevent residual contaminants from building up within glove box 2. Glove ports 6 are provided for accessing the interior of glove box 2, and a window 8 is included to allow observation of interior conditions. Various sealed access ports (not shown) may also be included for inserting and removing substances or for passing power, gas and vacuum lines, or other utilities into the interior of glove box 2.
FIG. 1B is an enlarged sectional side view of a portion of the glove box in FIG. 1A taken along line −1B-1B showing the zipper window assembly for holding and sealing window 8 in place. A gasket 10 of elastomeric material seats window 8 in the window aperture of glove box wall 4. Gasket 10 includes a first U-shaped channel 12 that surrounds the edges of the aperture in wall 4 and an opposing, second U-shaped channel 14 that surrounds the edges of window 8. A wedge member 16 is inserted into a slot 18 in the side of gasket 10 to compress gasket 10 and seal the medial section of gasket 10 between channels 12 and 14 against the peripheries of wall 4 and window 8. A stainless steel frame 20 may also be attached over the interface between wall 4 and window 8 on the interior of glove box 2 in order to protect gasket 10 from corrosion or, in the event of fire, from combustion of substances within glove box 2. Frame 20 is typically attached by welding to the inside of wall 4.
Another approach to a window assembly structure used in the prior art with enclosures for containing hazardous substances involves bolting a window directly to the exterior of the enclosures. FIG. 2 shows a cross-sectional view of such a window assembly mounted to the wall 22 of an enclosure constructed as a hot cell. As with glove box 2, wall 22 is formed from plates of stainless steel. To prevent radiation from escaping from the hot cell, the outside of wall 22 may be lined with a layer of lead 24 or a similar radiation-absorbing material. A window 26 is provided that is also formed from radiation-absorbing materials, such as with one or more sheets of glass and/or clear polycarbonates impregnated with lead. Depending on the radioactivity of the substances contained within the hot cell enclosure, window 26 may have a thickness of a few inches or more to prevent the escape of radiation. Window 26 is positioned over the aperture in wall 22 and held in place by a flanged stainless steel frame 28 attached with bolts 30 welded directly to wall 22. As seen in FIG. 2, a U-shaped gasket 32 of elastomeric material surrounds the edges of window 26 to seal the interfaces with wall 22 and frame 28.
While the above-described window assembly designs have been used extensively, they have inherent structural shortcomings that raise concerns in the construction and sealing of enclosures. The structure of the zipper-type window assembly, for example, may encounter problems when sealing against high or low interior pressure conditions. In situations where the pressure differential between the interior of the enclosure and the surrounding environment is substantial, failure of the gasket in a zipper window assembly has even led to windows being blown entirely out of their window apertures. The window assembly wherein a window is bolted to the exterior of an enclosure also presents problems, especially when it is necessary to modify or replace a window. When an enclosure has been used to contain highly toxic or radioactive substances, material forming the enclosure walls may become contaminated to the point that it may not be cut, drilled, ground, or otherwise reworked. Therefore, bolts welded to the enclosure walls that have been stripped or damaged cannot be replaced. Likewise, if a thicker window is required to accommodate increased radiation levels within the enclosure, larger bolt holes may not be drilled in the walls to receive bolts capable of holding the higher window mass.
Another problem with these window assembly designs is that they provide little or no reinforcement for the junction between a window and an enclosure wall. As previously described, enclosure walls are commonly formed from plates of stainless steel. The thickness of this material may vary widely, with a ⅝ inch thick plate of stainless steel having a tolerance of ±⅛ inch, for example. The plates of stainless steel may also be warped or twisted, especially for walls of large-sized enclosures. With the prior art window assemblies, such irregularities around a window aperture may make it difficult to achieve proper sealing or may translate stress into the window itself. It has, therefore, been necessary to planarize the plates of stainless steel around window apertures by welding or grinding prior to attaching a window. These processes are time consuming and difficult to perform, and cannot be carried out on enclosure surfaces that have been contaminated by highly toxic or radioactive substances.
In view of the foregoing, a need exists for improved window assemblies that eliminate sealing problems and that simplify the installation, removal, and modification of enclosure windows.